Communication Systems

ABSTRACT

A transmission method for use in a two-hop wireless communication system is provided. The system includes a source apparatus, a destination apparatus and an intermediate apparatus. The source apparatus is operable to transmit information along two links forming a communication path extending from the source apparatus to the destination apparatus via the intermediate apparatus. The intermediate apparatus is operable to receive information from the source apparatus and to transmit the received information to the destination apparatus. The system has access to a time-frequency format for use in assigning available transmission frequency bandwidth during a discrete transmission interval, said format defining a plurality of transmission windows within such an interval. Each window occupies a different part of that interval and has a frequency bandwidth profile within said available transmission frequency bandwidth over its part of that interval. Furthermore, each said window is assignable for such a transmission interval to said source or intermediate apparatus for use in transmission. The transmission method for use in this system includes employing said format to transmit information along the path as two successive transmission signals, link by link, said signals being transmitted using different transmission windows of a particular such transmission interval.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following applications, each of which isincorporated herein by reference:

COMMUNICATION SYSTEMS, Attorney Docket 017071.0125, application Ser. No.______, filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, Attorney Docket 017071.0126, application Ser. No.______, filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, Attorney Docket 017071.0127, application Ser. No.______ , filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, Attorney Docket 017071.0129, application Ser. No.______, filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, Attorney Docket 017071.0130, application Ser. No.______, filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, Attorney Docket 017071.0131, application Ser. No.______, filed Aug. 17, 2007 and currently pending;

COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616478.4,filed on Aug. 18, 2006;

COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616475.0,filed on Aug. 18, 2006; and

COMMUNICATION SYSTEMS, United Kingdom Application No. GB 0616476.8,filed on Aug. 18, 2006.

RELATED APPLICATION

This application claims foreign priority benefits under 35 U.S.C. § 119of United Kingdom Application No. GB 0616477.6, filed on Aug. 18, 2006,entitled “Communication Systems”.

TECHNICAL FIELD

This invention relates in general to communication systems, and moreparticularly to a frame structure for a multihop communication system.

OVERVIEW

Currently there exists interest in the use of multihop techniques inpacket based radio and other communication systems, where it ispurported that such techniques will enable both extension in coveragerange and increase in system capacity (throughput).

In a multi-hop communication system, communication signals are sent in acommunication direction along a communication path (C) from a sourceapparatus to a destination apparatus via one or more intermediateapparatuses. FIG. 4 illustrates a single-cell two-hop wirelesscommunication system comprising a base station BS (known in the contextof 3G communication systems as “node-B” NB) a relay node RN (also knownas a relay station RS) and a user equipment UE (also known as mobilestation MS). In the case where signals are being transmitted on thedownlink (DL) from a base station to a destination user equipment (UE)via the relay node (RN), the base station comprises the source station(S) and the user equipment comprises the destination station (D). In thecase where communication signals are being transmitted on the uplink(UL) from a user equipment (UE), via the relay node, to the basestation, the user equipment comprises the source station and the basestation comprises the destination station. The relay node is an exampleof an intermediate apparatus (I) and comprises: a receiver, operable toreceive data from the source apparatus; and a transmitter, operable totransmit this data, or a derivative thereof, to the destinationapparatus.

Simple analogue repeaters or digital repeaters have been used as relaysto improve or provide coverage in dead spots. They can either operate ina different transmission frequency band from the source station toprevent interference between the source transmission and the repeatertransmission, or they can operate at a time when there is notransmission from the source station.

FIG. 5 illustrates a number of applications for relay stations. Forfixed infrastructure, the coverage provided by a relay station may be“in-fill” to allow access to the communication network for mobilestations which may otherwise be in the shadow of other objects orotherwise unable to receive a signal of sufficient strength from thebase station despite being within the normal range of the base station.“Range extension” is also shown, in which a relay station allows accesswhen a mobile station is outside the normal data transmission range of abase station. One example of in-fill shown at the top right of FIG. 5 ispositioning of a nomadic relay station to allow penetration of coveragewithin a building that could be above, at, or below ground level.

Other applications are nomadic relay stations which are brought intoeffect for temporary cover, providing access during events oremergencies/disasters. A final application shown in the bottom right ofFIG. 5 provides access to a network using a relay positioned on avehicle.

Relays may also be used in conjunction with advanced transmissiontechniques to enhance gain of the communications system as explainedbelow.

It is known that the occurrence of propagation loss, or “pathloss”, dueto the scattering or absorption of a radio communication as it travelsthrough space, causes the strength of a signal to diminish. Factorswhich influence the pathloss between a transmitter and a receiverinclude: transmitter antenna height, receiver antenna height, carrierfrequency, clutter type (urban, sub-urban, rural), details of morphologysuch as height, density, separation, terrain type (hilly, flat). Thepathloss L (dB) between a transmitter and a receiver can be modeled by:L=b+10nlogd  (A)Where d (meters) is the transmitter-receiver separation, b(db) and n arethe pathloss parameters and the absolute pathloss is given byl=10^((L/10)).

The sum of the absolute path losses experienced over the indirect linkSI+ID may be less than the pathloss experienced over the direct link SD.In other words it is possible for:L(SI)+L(ID)<L(SD)  (B)

Splitting a single transmission link into two shorter transmissionsegments therefore exploits the non-linear relationship between pathlossverses distance. From a simple theoretical analysis of the pathlossusing equation (A), it can be appreciated that a reduction in theoverall pathloss (and therefore an improvement, or gain, in signalstrength and thus data throughput) can be achieved if a signal is sentfrom a source apparatus to a destination apparatus via an intermediateapparatus (e.g. relay node), rather than being sent directly from thesource apparatus to the destination apparatus. If implementedappropriately, multi-hop communication systems can allow for a reductionin the transmit power of transmitters which facilitate wirelesstransmissions, leading to a reduction in interference levels as well asdecreasing exposure to electromagnetic emissions. Alternatively, thereduction in overall pathloss can be exploited to improve the receivedsignal quality at the receiver without an increase in the overallradiated transmission power required to convey the signal.

Multi-hop systems are suitable for use with multi-carrier transmission.In a multi-carrier transmission system, such as FDM (frequency divisionmultiplex), OFDM (orthogonal frequency division multiplex) or DMT(discrete multi-tone), a single data stream is modulated onto N parallelsub-carriers, each sub-carrier signal having its own frequency range.This allows the total bandwidth (i.e. the amount of data to be sent in agiven time interval) to be divided over a plurality of sub-carriersthereby increasing the duration of each data symbol. Since eachsub-carrier has a lower information rate, multi-carrier systems benefitfrom enhanced immunity to channel induced distortion compared withsingle carrier systems. This is made possible by ensuring that thetransmission rate and hence bandwidth of each subcarrier is less thanthe coherence bandwidth of the channel. As a result, the channeldistortion experienced on a signal subcarrier is frequency independentand can hence be corrected by a simple phase and amplitude correctionfactor. Thus the channel distortion correction entity within amulticarrier receiver can be of significantly lower complexity of itscounterpart within a single carrier receiver when the system bandwidthis in excess of the coherence bandwidth of the channel.

Orthogonal frequency division multiplexing (OFDM) is a modulationtechnique that is based on FDM. An OFDM system uses a plurality ofsub-carrier frequencies which are orthogonal in a mathematical sense sothat the sub-carriers' spectra may overlap without interference due tothe fact they are mutually independent. The orthogonality of OFDMsystems removes the need for guard band frequencies and therebyincreases the spectral efficiency of the system. OFDM has been proposedand adopted for many wireless systems. It is currently used inAsymmetric Digital Subscriber Line (ADSL) connections, in some wirelessLAN applications (such as WiFi devices based on the IEEE 802.11a/gstandard), and in wireless MAN applications such as WiMAX (based on theIEEE 802.16 standard). OFDM is often used in conjunction with channelcoding, an error correction technique, to create coded orthogonal FDM orCOFDM. COFDM is now widely used in digital telecommunications systems toimprove the performance of an OFDM based system in a multipathenvironment where variations in the channel distortion can be seenacross both subcarriers in the frequency domain and symbols in the timedomain. The system has found use in video and audio broadcasting, suchas DVB and DAB, as well as certain types of computer networkingtechnology.

In an OFDM system, a block of N modulated parallel data source signalsis mapped to N orthogonal parallel sub-carriers by using an InverseDiscrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form asignal known as an “OFDM symbol” in the time domain at the transmitter.Thus, an “OFDM symbol” is the composite signal of all N sub-carriersignals. An OFDM symbol can be represented mathematically as:$\begin{matrix}{{{x(t)} = {\frac{1}{\sqrt{N}}{\sum\limits_{n = 0}^{N - 1}{c_{n} \cdot {\mathbb{e}}^{j\quad 2\quad\pi\quad n\quad\Delta\quad f\quad t}}}}},{0 \leq t \leq T_{s}}} & (1)\end{matrix}$where Δf is the sub-carrier separation in Hz, Ts=1/Δf is symbol timeinterval in seconds, and c_(n) are the modulated source signals. Thesub-carrier vector in (1) onto which each of the source signals ismodulated cεC_(n), c=(c₀, c₁ . . . c_(N−1)) is a vector of Nconstellation symbols from a finite constellation. At the receiver, thereceived time-domain signal is transformed back to frequency domain byapplying Discrete Fourier Transform (DFT) or Fast Fourier Transform(FFT) algorithm.

OFDMA (Orthogonal Frequency Division Multiple Access) is a multipleaccess variant of OFDM. It works by assigning a subset of sub-carriers,to an individual user. This allows simultaneous transmission fromseveral users leading to better spectral efficiency. However, there isstill the issue of allowing bi-directional communication, that is, inthe uplink and download directions, without interference.

In order to enable bi-directional communication between two nodes, twowell known different approaches exist for duplexing the two (forward ordownload and reverse or uplink) communication links to overcome thephysical limitation that a device cannot simultaneously transmit andreceive on the same resource medium. The first, frequency divisionduplexing (FDD), involves operating the two links simultaneously but ondifferent frequency bands by subdividing the transmission medium intotwo distinct bands, one for forward link and the other for reverse linkcommunications. The second, time division duplexing (TDD), involvesoperating the two links on the same frequency band, but subdividing theaccess to the medium in time so that only the forward or the reverselink will be utilizing the medium at any one point in time. Bothapproaches (TDD & FDD) have their relative merits and are both well usedtechniques for single hop wired and wireless communication systems. Forexample the IEEE 802.16 standard incorporates both an FDD and TDD mode.As an example, FIG. 6 illustrates the single hop TDD frame structureused in the OFDMA physical layer mode of the IEEE 802.16 standard(WiMAX).

Each frame is divided into DL and UL subframes, each being a discretetransmission interval. They are separated by Transmit/Receive andReceive/Transmit Transition Guard interval (TTG and RTG respectively).Each DL subframe starts with a preamble followed by the Frame ControlHeader (FCH), the DL-MAP, and the UL-MAP.

The FCH contains the DL Frame Prefix (DLFP) to specify the burst profileand the length of the DL-MAP. The DLFP is a data structure transmittedat the beginning of each frame and contains information regarding thecurrent frame; it is mapped to the FCH.

Simultaneous DL allocations can be broadcast, multicast and unicast andthey can also include an allocation for another BS rather than a servingBS. Simultaneous ULs can be data allocations and ranging or bandwidthrequests.

SUMMARY OF EXAMPLE EMBODIMENTS

In accordance with one embodiment of the present invention, atransmission method for use in a two-hop wireless communication systemis provided. The system includes a source apparatus, a destinationapparatus and an intermediate apparatus. The source apparatus isoperable to transmit information along two links forming a communicationpath extending from the source apparatus to the destination apparatusvia the intermediate apparatus. The intermediate apparatus is operableto receive information from the source apparatus and to transmit thereceived information to the destination apparatus. The system has accessto a time-frequency format for use in assigning available transmissionfrequency bandwidth during a discrete transmission interval, said formatdefining a plurality of transmission windows within such an interval.Each window occupies a different part of that interval and has afrequency bandwidth profile within said available transmission frequencybandwidth over its part of that interval. Furthermore, each said windowis assignable for such a transmission interval to said source orintermediate apparatus for use in transmission. The transmission methodfor use in this system includes employing said format to transmitinformation along the path as two successive transmission signals, linkby link, said signals being transmitted using different transmissionwindows of a particular such transmission interval.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 shows a frame structure;

FIG. 2 shows node activity within each zone;

FIG. 3 shows an example of zone usage within one cell;

FIG. 4 shows a single-cell two-hop wireless communication system;

FIG. 5 shows applications of relay stations; and

FIG. 6 shows a single hop TDD frame structure used in the OFDMA physicallayer mode of the IEEE 802.16 standard.

DETAILED DESCRIPTION

When a node is required to support two independent links to twodifferent nodes, e.g. a relay station communicating with a base stationand a mobile, the existing TDD or FDD frame structures require somemodification in order to make realization of the relay practical.Particular embodiments of the invention provide a frame structure(format) for a multihop communication system that is an extension of thestandard TDD frame structure.

One proposed frame structure is designed for the case that the controlinformation originating from the head node that controls the overallmedium access is receivable by all subordinate nodes operating in thenetwork. It is further designed in a manner that enables legacy singlehop TDD mobile devices that have no knowledge of a relay station tooperate within the new relaying enabled system.

If control information is not receivable from the head node (or sourceapparatus) then an extra frame period is required for two-hoptransmission. This is because control information sent by the sourceapparatus to the intermediate apparatus cannot then be received by thedestination apparatus in the same frame. The destination apparatus(especially a legacy apparatus) will be designed to receive such controlinformation at the beginning of the frame and therefore an extra frameperiod is required for the intermediate apparatus to transmit thecontrol information on to the source at the beginning of the frame (inthe preamble) and then transmit the data. Thus a frame latency of 1 isincurred.

An example frame structure is shown in FIG. 1. It is composed of anumber of transmission and reception zones for both the downlink anduplink sub-frames. The zone types are either:

-   B Broadcast of control related information such as: synchronization    sequences, commands, information and details of the structure or    layout of the frame.-   C Dedicated control information that is transmitted in a    non-broadcast zone (i.e. either to individual or a group of    receivers)-   T Dedicated user-data (transport) transmission

The 9 different zones identified in FIG. 1 are described in Table 1.TABLE 1 Description of the zones. Zone Number Label Description 1 PPreamble or synchronization sequence transmissions for cellidentification 2 MAP Frame format description (zone boundaries,allocations within the zones, etc) 3 BS-RS/BS- BS to RS transmissionzone. Can also be MS used for BS to MS transmission if spatial divisionmultiple access is supported (i.e. the same transmission resource can beused to communicate with more than one entity) 4 BS-MS BS to MStransmission zone. RS is not active during this period, it is processingany received information and turning around prior to transmission. 5BS-MS/RS- RS to MS transmission zone. Can also be MS used by the BS totransmitted to MSs that do not experience significant levels ofinterference from RS transmissions. 6 MS-BS/MS- MS control informationtransmission zone. RS Information can be received by both the RS and theBS. Control information can be information or requests from the MS. 7MS-BS/MS- MS to RS transmission zone. Can also be RS used by MSs who donot cause interference to the RS to transmit to the BS. 8 MS-BS MS to BStransmission zone. RS is not actively transmitting or receiving duringthis period; it is processing any received information prior to turningaround. 9 RS-BS/MS- RS to BS transmission zone. Can also be BS used forMS to BS transmission if spatial division multiple access is supported(i.e. the same transmission resource can be used to communicate withmore than one entity)

FIG. 2 illustrates a preferred operation of the BS, RS and MS in termsof its activity within each of the zones described in Table 1.

FIG. 3 indicates one particular realization of the proposed framestructure in terms of how different user types may be allocated fortransmission or reception within the various zone types.

In this case there are five link types identified (A-E), as illustratedin FIG. 3. A description of the zones that are used in this example isgiven in Table 2. TABLE 2 Description of example of zone usage withinone cell. DL Zone UL Zone Link Usage Usage Comments (A) (1), (2), (5)(6), (7) MS and RS are spatially separated and therefore significantinterference isolation exists. User does not support SDMA. (B) (1), (2),(3) (6), (9) MS and RS are spatially separated and therefore significantinterference isolation exists. User does support SDMA. (C) (1), (2),(3), (6), (7), (9) RS receives data in (3) and (7) and (5) thentransmits in (5) and (9) thereby enabling in-frame relaying. (D) (1),(2), (5) (6), (7) MS communicates with BS via RS. Transmission to the RShappens at the beginning of the UL subframe (7) to allow sufficient RSrelay processing time. (E) (1), (2), (4) (6), (8) MSs that communicatedirectly with the BS that are not isolated from the RS use zones (4) &(8) to prevent RS interference from impairing link performance.

One of the advantages of adopting the proposed frame structure ofinvention embodiments is that the BS can make use of all of thetransmission resource all of the time to communicate with the RS and MSnodes in the network. This is enabled by reusing the transmissionresource used on the RS to MS link for BS to MS communications. In orderto effect this, and prevent such a reuse approach from causing excessinterference, the BS must ensure that the users it communicates withinthis reuse zone (i.e. zones (5) & (9)) are sufficiently isolated fromthe users communicating with the RS. Thus, the BS essentially requires amechanism to decide whether the users with which it communicates shouldbe in the reuse zone (i.e. zones (5) & (9)) or the normal zone (i.e.zones (4) & (8)).

There are numerous algorithms that can be used to form such a mechanism.A few examples are listed below:

-   1. Ask the MS to perform a carrier-to-interference-plus-noise (CINR)    measurement on the BS transmission during the reuse zone and during    the normal zone. If the CINR is much higher in the normal zone then    allocate the user to the normal zone. If the CINR is similar, then    allocate the user to the reuse zone.-   2. Start with all users in the normal zone. If the normal zone    becomes fully loaded and cannot accommodate more users without the    risk of imposing a reduction on the quality of service new and    existing users will experience, then identify candidate users to    move from the normal zone to the reuse zone. If subsequently the    reported CINR for a user communicating with the BS in the reuse zone    falls below a particular threshold then move that to the normal    zone.    In summary, benefits of particular embodiments may include:-   Enabling the construction and operation of simple, low cost relays    that do not need to generate any control information or perform    scheduling-   Maximize spectral efficiency by ensuring that the BS does not have    any time in the frame when it is idle-   Minimize latency by enabling two-hop relaying to occur within one    frame-   Enable the system to potentially provide transparent operation to a    legacy single-hop TDD users-   The possibility to further improve spectral efficiency through using    SDMA based techniques to enable the same transmission resource    (frequency & time) to be used between the BS and the RSs and MSs    within a cell.-   Provide a mechanism to enable reuse of the RS-MS communication zone    by the BS to communicate directly with MSs that will not cause a    degradation in RS-MS link performance by performing such    communications.

Embodiments of the present invention may be implemented in hardware, oras software modules running on one or more processors, or on acombination thereof. That is, those skilled in the art will appreciatethat a microprocessor or digital signal processor (DSP) may be used inpractice to implement some or all of the functionality of a transmitterembodying the present invention. The invention may also be embodied asone or more device or apparatus programs (e.g. computer programs andcomputer program products) for carrying out part or all of any of themethods described herein. Such programs embodying the present inventionmay be stored on computer-readable media, or could, for example, be inthe form of one or more signals. Such signals may be data signalsdownloadable from an Internet website, or provided on a carrier signal,or in any other form.

Although the present invention has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

1. A transmission method for use in a two-hop wireless communicationsystem, the system comprising a source apparatus, a destinationapparatus and an intermediate apparatus, said source apparatus beingoperable to transmit information along two links forming a communicationpath extending from the source apparatus to the destination apparatusvia the intermediate apparatus, and the intermediate apparatus beingoperable to receive information from the source apparatus and totransmit the received information to the destination apparatus, thesystem having access to a time-frequency format for use in assigningavailable transmission frequency bandwidth during a discretetransmission interval, said format defining a plurality of transmissionwindows within such an interval, each window occupying a different partof that interval and having a frequency bandwidth profile within saidavailable transmission frequency bandwidth over its part of thatinterval, each said window being assignable for such a transmissioninterval to said source or intermediate apparatus for use intransmission, the method comprising: employing said format to transmitinformation along the path as two successive transmission signals, linkby link, said signals being transmitted using different transmissionwindows of a particular such transmission interval.
 2. The transmissionmethod according to claim 1, wherein the frequency bandwidth profiles ofat least two of said transmission windows encompass a common part of theavailable transmission frequency bandwidth.
 3. The transmission methodaccording to claim 1, wherein the frequency bandwidth profiles of atleast two said transmission windows extend over substantially the entiretransmission frequency bandwidth for the respective interval parts. 4.The transmission method according to claim 1, further comprising, priorto said transmission, employing said format to assign a particulartransmission window of the particular transmission interval to thesource apparatus for transmission of the information to the intermediateapparatus, and to assign a subsequent transmission window of theparticular transmission interval to the intermediate apparatus fortransmission of the information to the destination apparatus.
 5. Thetransmission method according to claim 1, further comprising employingthe format to assign a control window to the source apparatus fortransmission of control information to the intermediate apparatus. 6.The transmission method according to claim 4, further comprisingemploying the format to assign a control window to the source apparatusfor transmission of control information to the intermediate apparatuswherein said control window occupies a part of the particulartransmission interval preceding the part of the particular transmissioninterval occupied by the particular transmission window.
 7. Thetransmission method according to claim 4, wherein said particular andsubsequent transmission windows of the particular transmission intervalare either side in time of a further transmission window of thatinterval.
 8. The transmission method according to claim 7, furthercomprising performing processing in said intermediate apparatus duringthe part of the particular transmission interval corresponding to thefurther transmission window, so as to configure the information fortransmission in the subsequent transmission window based upon theinformation received in the particular transmission window.
 9. Thetransmission method according to claim 7, wherein said communicationpath is an indirect communication path, and wherein the system comprisesat least a further destination apparatus, and wherein said sourceapparatus is operable to transmit information directly to the or eachfurther destination apparatus along a corresponding single link forminga direct communication path.
 10. The transmission method according toclaim 9, further comprising employing said further transmission windowto transmit information from the source apparatus to a said furtherdestination apparatus along such a direct communication path, so thatinformation is transmitted from the source apparatus during saidprocessing in the intermediate apparatus.
 11. The transmission methodaccording to claim 9, further comprising employing said particulartransmission window to transmit information from the source apparatus toa said further destination apparatus along such a direct communicationpath, so that information is transmitted from the source apparatus toboth said intermediate apparatus and said further destination apparatusduring the part of the particular transmission interval corresponding tothe particular transmission window.
 12. The transmission methodaccording to claim 9, further comprising employing said subsequenttransmission window to transmit information from the source apparatus toa said further destination apparatus along such a direct communicationpath, so that information is transmitted from said intermediateapparatus to said destination apparatus and from the source apparatus tosaid further destination apparatus during the part of the subsequenttransmission interval corresponding to the subsequent transmissionwindow.
 13. The transmission method according to claim 7, wherein saidcommunication path is an indirect communication path, and wherein thesystem comprises at least a further source apparatus, and wherein thefurther source apparatus is operable to transmit information directly tothe destination apparatus along a corresponding single link forming adirect communication path.
 14. The transmission method according toclaim 13, further comprising employing said further transmission windowto transmit information from a said further source apparatus to saiddestination apparatus along such a direct communication path, so thatinformation is transmitted from that further source apparatus duringsaid processing in the intermediate apparatus.
 15. The transmissionmethod according to claim 13, further comprising employing saidparticular transmission window to transmit information from a saidfurther source apparatus to said destination apparatus along such adirect communication path, so that information is transmitted from thesource apparatus to said intermediate apparatus and from that furthersource apparatus to said further destination apparatus during the partof the particular transmission interval corresponding to the particulartransmission window.
 16. The transmission method according to claim 13,further comprising employing said subsequent transmission window totransmit information from a said further source apparatus to saiddestination apparatus along such a direct communication path, so thatinformation is transmitted from said intermediate apparatus to saiddestination apparatus and from that further source apparatus to saiddestination apparatus during the part of the subsequent transmissioninterval corresponding to the subsequent transmission window.
 17. Thetransmission method according to claim 7, comprising employing a spacedivision multiple access technique in one or more of said transmissionwindows of the particular transmission interval.
 18. The transmissionmethod according to claim 1, wherein the time-frequency format is aformat for a downlink or uplink sub-frame in a time-division-duplexcommunication system.
 19. The transmission method according to claim 1,wherein said system is an OFDM or OFDMA system, and wherein thetime-frequency format is a format for an OFDM or OFDMA downlink oruplink sub-frame of an OFDM or OFDMA time-division-duplex frame.
 20. Thetransmission method according to claim 1, wherein each said discretetransmission interval is a sub-frame period.
 21. The transmission methodaccording to claim 1, wherein each said transmission window comprises aregion in an OFDM or OFDMA frame structure.
 22. The transmission methodaccording to claim 1, wherein each said transmission window comprises azone in an OFDM or OFDMA frame structure.
 23. The transmission methodaccording to claim 1, wherein the source apparatus is a base station.24. The transmission method according to claim 1, wherein the sourceapparatus is a user terminal.
 25. The transmission method according toclaim 1, wherein the destination apparatus is a base station.
 26. Thetransmission method according to claim 1, wherein the destinationapparatus is a user terminal.
 27. The transmission method according toclaim 1, wherein the intermediate apparatus is a relay station.
 28. Atransmission method for use in a two-hop wireless communication system,the system comprising a source apparatus, a destination apparatus and anintermediate apparatus, said source apparatus being operable to transmitinformation along two links forming a communication path extending fromthe source apparatus to the destination apparatus via the intermediateapparatus, and the intermediate apparatus being operable to receiveinformation from the source apparatus and to transmit the receivedinformation to the destination apparatus, the system having access to atime-frequency format for use in assigning available transmissionfrequency bandwidth during a discrete transmission interval, said formatdefining a plurality of transmission windows within such an interval,each window occupying a different part of that interval and having afrequency bandwidth profile within said available transmission frequencybandwidth over its part of that interval, each said window beingassignable for such a transmission interval to said source orintermediate apparatus for use in transmission, the method comprising:employing said format to transmit data and control information togetheras a transmission signal along the link from the source apparatus to theintermediate apparatus and to transmit data information as atransmission signal along the link from the intermediate apparatus tothe destination apparatus, said signals being transmitted usingrespective transmission windows of two such transmission intervals. 29.A two-hop wireless communication system, the system comprising: a sourceapparatus, a destination apparatus and an intermediate apparatus, saidsource apparatus being operable to transmit information along two linksforming a communication path extending from the source apparatus to thedestination apparatus via the intermediate apparatus, and theintermediate apparatus being operable to receive information from thesource apparatus and to transmit the received information to thedestination apparatus; format-access means operable to access atime-frequency format for use in assigning available transmissionfrequency bandwidth during a discrete transmission interval, said formatdefining a plurality of transmission windows within such an interval,each window occupying a different part of that interval and having afrequency bandwidth profile within said available transmission frequencybandwidth over its part of that interval, each said window beingassignable for such a transmission interval to said source orintermediate apparatus for use in transmission; and transmission meansoperable to employ said format to transmit information along the relayedpath as two successive transmission signals, link by link, usingdifferent transmission windows of a particular such transmissioninterval.
 30. Software embodied in a computer-readable medium andoperable, when executed on one or more computing devices of a two-hopwireless communication system, to cause the system to carry out atransmission method, the system comprising a source apparatus, adestination apparatus and an intermediate apparatus, said sourceapparatus being operable to transmit information along two links forminga communication path extending from the source apparatus to thedestination apparatus via the intermediate apparatus, and theintermediate apparatus being operable to receive information from thesource apparatus and to transmit the received information to thedestination apparatus, the system having access to a time-frequencyformat for use in assigning available transmission frequency bandwidthduring a discrete transmission interval, said format defining aplurality of transmission windows within such an interval, each windowoccupying a different part of that interval and having a frequencybandwidth profile within said available transmission frequency bandwidthover its part of that interval, each said window being assignable forsuch a transmission interval to said source or intermediate apparatusfor use in transmission, the method comprising: employing said format totransmit information along the relayed path as two successivetransmission signals, link by link, said signals being transmitted usingdifferent transmission windows of a particular such transmissioninterval.
 31. An intermediate apparatus for use in a two-hop wirelesscommunication system, the system further comprising a source apparatus,and a destination apparatus, said source apparatus being operable totransmit information along two links forming a communication pathextending from the source apparatus to the destination apparatus via theintermediate apparatus, and the intermediate apparatus being operable toreceive information from the source apparatus and to transmit thereceived information to the destination apparatus, the intermediateapparatus comprising: a format-accessor operable to access atime-frequency format for use in assigning available transmissionfrequency bandwidth during a discrete transmission interval, said formatdefining a plurality of transmission windows within such an interval,each window occupying a different part of that interval and having afrequency bandwidth profile within said available transmission frequencybandwidth over its part of that interval, each said window beingassignable for such a transmission interval to said source or saidintermediate apparatus for use in transmission; and a transceiveroperable to employ said format for one such transmission interval toreceive information in an available transmission window of said intervaland to transmit said information in a later available transmissionwindow during the same said transmission interval such that saidinformation passes along the two links in a single transmissioninterval.
 32. A transmission method for use in an intermediate apparatusof a two-hop wireless communication system, the system furthercomprising a source apparatus and a destination apparatus, said sourceapparatus being operable to transmit information along two links forminga communication path extending from the source apparatus to thedestination apparatus via the intermediate apparatus, and theintermediate apparatus being operable to receive information from thesource apparatus and to transmit the received information to thedestination apparatus, the intermediate apparatus having access to atime-frequency format for use in assigning available transmissionfrequency bandwidth during a discrete transmission interval, said formatdefining a plurality of transmission windows within such an interval,each window occupying a different part of that interval and having afrequency bandwidth profile within said available transmission frequencybandwidth over its part of that interval, each said window beingassignable for such a transmission interval to said source orintermediate apparatus for use in transmission, the method comprising:employing said format for one such transmission method to receiveinformation in an available window of said interval and to transmit saidinformation in a later available window during the same transmissioninterval such that said information passes along the two links in asingle transmission interval.
 33. Software embodied on acomputer-readable medium and operable, which when executed on acomputing device of an intermediate apparatus in a two-hop wirelesscommunication system, to cause the intermediate apparatus to carry out atransmission method, the system further comprising a source apparatusand a destination apparatus, said source apparatus being operable totransmit information along two links forming a communication pathextending from the source apparatus to the destination apparatus via theintermediate apparatus, and the intermediate apparatus being operable toreceive information from the source apparatus and to transmit thereceived information to the destination apparatus, the intermediateapparatus having access to a time-frequency format for use in assigningavailable transmission frequency bandwidth during a discretetransmission interval, said format defining a plurality of transmissionwindows within such an interval, each window occupying a different partof that interval and having a frequency bandwidth profile within saidavailable transmission frequency bandwidth over its part of thatinterval, each said window being assignable for such a transmissioninterval to said source or intermediate apparatus for use intransmission, the method comprising: employing said format for one suchtransmission method to receive information in an available window ofsaid interval and to transmit said information in a later availablewindow during the same transmission interval such that said informationpasses along the two links in a single transmission interval.